Hmm, I've used a number in the past and have gone on lots of dates and even got some girlfriends from them.
Myspace - Met my last g/f off there, just added her as a friend, she commented me, so we exchanged comments back and forth, then emails, then we talked on AIM, met up one day and it went from there. Overall most of the girls on there seem kind of airheaded though, its truly the cesspool of the internet

It's free though.
eHarmony - It's expensive and honestly I didn't really care much at all for any of the girls I met through it. They were all very dull and we hardly had anything in common... maybe I just fucked up in the long survey you have to take, or maybe I'm just a boring person myself
Match - Pretty much the most basic matching site... I've met a number of girls off here, no relationships yet... The only problem is there are so many people and its hard to tell who has an account, who doesn't... like maybe 1 in 10 people will respond to your emails, that's not your fault, its just because most people will sign up, and forget they have an account. This might be a good choice... Seriously though, after the girl from Myspace broke up with me, I made an account, went on a "winking" spree (you can "wink" at girls to let them know you're interested, w/o emailing them... but most girls don't like this, because its lazy, and would prefer a thoughtful email) and within 3 days I already had a number of dates lined up. Had to actually make a database on my computer of all the girls, like their hometowns, their interests, and stuff, just so I wouldn't get all mixed up :lol
OKCupid - Its free and there's not too many people on here... but I've had more luck with girls who contact me first, than any other site. Actually the girl I'm interested in now emailed me from there because she liked my profile (we hung out quite a number of times, and I want to make a move before I go off to E3). Another one of my friends met a girl he really likes off there, and one of my ex's (who I'm still good friends with) met her current b/f off there.